This invention relates to charging capacitive loads. More particularly, this invention relates to charging capacitive loads in photoflash systems.
In conventional photoflash systems, fixed frequency switching power supply topologies are typically used to provide power to a capacitive load. For example, in fixed frequency applications, a portion of the period associated with the frequency can be used to turn a power switch (e.g., transistor) ON and another portion of the period can be used to turn the switch OFF. A ratio of ON-time TON versus OFF-time TOFF can be set to adjust the duty ratio applied to the power switch. During ON-time, the power switch is activated and then during OFF-time, the power switch is OFF. The TOFF/TON ratio can be adjusted to provide the appropriate power to the capacitive load during the switching cycle of the switching power supply. Typical DC-to-DC converters, for example, employ this technique. Therefore, under varying load conditions or output voltage requirements, conventional switching power supply topologies can adjust the TOFF/TON ratio to meet output voltage and load requirements.
This approach as it relates to photoflash systems, however, has several potential problems. One problem is that the photoflash capacitor voltage can vary continuously from, for example, 0V at the start of a charging cycle to 300V at the end of the charging cycle. This wide variation in voltages can put demands that are impractical to implement on conventional power switching supplies. For example, some conventional switching power supplies may not have the capability to adjust the TOFF/TON ratio to provide power to charge output capacitor loads that vary over a wide voltage range.
Another potential problem that may occur with conventional power switching supplies is that the output voltage feedback mechanism used to monitor the output voltage can be a source of constant power dissipation. For example, a feedback mechanism may include a resistor divider coupled between the output capacitor load and ground. During operation, this coupling exhibits an I2R power loss. Furthermore, several tens of microamps may be required to be conducted in the resistor divider to minimize the affect of finite input impedance of the feedback mechanism. In addition, when the conventional switching supply operates to maintain a relatively high output voltage (e.g., 300V), the feedback mechanism can dissipate several milliwatts. Since it is desirable to maintain the capacitor voltage at flash ready status, the feedback mechanism has to constantly monitor the capacitor voltage to ensure that the proper voltage is maintained, thus creating an undesirable long term power loss.
Another problem that can occur with conventional switching power supplies is that the switching action required to obtain the proper output voltage cannot be stopped. Instead, the conventional switching power supply continuously adjusts the TOFF/TON ratio to maintain a constant output voltage relative to a given load. In other words, the conventional switching power supply continues to supply power to the load even when the desired capacitor voltage has been reached. This can add additional power losses that reduce the efficiency of conventional photoflash systems.